Fiber optic data links are being given serious consideration as a replacement for electrical links for a number of reasons: wider band width, improved immunity to electromagnetic interference, electrical isolation between the interconnected equipment, and the like. In its simplest form, such a data link utilizes a junction laser or light-emitting diode (LED), for example, as the transmitting element which is modulated between zero or near zero intensity and some predetermined peak light amplitude. The modulated light signal is then processed in an optical receiver where it is converted into an electrical signal which is decoded either by a threshold detection circuit or, in the event that the duty cycle averages 50 percent, by a zero-crossing detector (which is particularly suitable for use with a Manchester data encoding scheme). However, as discussed in U.S. Pat. No. 4,236,256, entitled "Asynchronous Data Receiver," especially in the case of asynchronous systems, it is preferable to modulate the light amplitude of the laser or LED light source such that a transitionally encoded three-level optical signal results. In such a system the light amplitude has associated therewith a dc light amplitude, L.sub.0, from which there may be upward transitions to a light amplitude double the normal dc amplitude or downward to a light amplitude effectively equal to zero. At the receiving end of the optical data link, it is possible to convert the light signal to an equivalent bipolar electrical signal by means of a suitable photodetector utilizing threshold detector circuits and logic circuits.
U.S. Pat. No. 3,714,437, entitled "Optical Communication System With PCM Encoding With Plural Discrete And Equally Spaced Intensity Levels," discusses the use of an N-nary PCM format where N is a positive energy greater than two and in which the intensity of each optical pulse may assume any one of N spaced apart levels. In particular, this Patent teaches the desirability of spacing the various intensity levels so as to minimize errors caused by the overlapping of intensity distribution associated with the different levels so as to maximize the information rate capability of the system. In such an optimized system, the discrete levels are not equally spaced apart but are rather determined by the formula ##EQU1## where j equals 1, 2, . . . N. In the case of a tri-level code, the three such levels would be zero, 1/4 and 1.
Military Standard MIL-STD-1553B dated Sept. 21, 1978 and entitled "Aircraft Internal Time Division Command/Response Multiplex Data Bus" is directed specifically to a class of electrical data buses that have been developed especially for avionics use and defines, in addition to the specific hardware characteristics of the cable and the manner in which it is coupled to the individual terminals and controllers, operational characteristics such as the use of Manchester II biphase level encoding at a transmission bit rate of one megabit per second, and also a command synch waveform. The Standard also specifies the format of the various commands to which the individual terminals and controllers are to respond. In response to specific commands issued by a designated bus controller, the individual remote terminals may exchange information with the controller or information transmitted by a first designated remote terminal may be received by a second designated remote terminal. The protocol ensures that at most only one terminal or controller will be transmitting data at any given time; when no data is being transmitted, the voltage on the line remains at zero.
Bearing the above prior art in mind, it is a primary object of the present invention to develop a fiber optic data bus analogous to the electrical bus specified by the above-referenced Military Standard 1553B such that the fiber optic data bus and the electrical data bus may be utilized simultaneously in the same system so as to permit the individual terminals to communicate with each other and with a common bus controller regardless of whether both are associated with the electrical bus or with the fiber optic bus or with one on each type of bus; that is to say, the physical differences between the electrical bus and the optical bus are "transparent" to the individual terminals/controllers and the two types of buses may be coupled to one another to form a hybrid type of data bus without any degradation in performance.
It is a more specific objective to provide such a hybrid system that has the additional advantages of improved reliability and a high degree of redundancy.